1. Field of the Invention
The present invention relates to a photoconductive liquid crystal light valve for use in a projection type liquid crystal display apparatus.
2. Background of the Invention
FIG. 1 shows an example of a projection type liquid crystal display apparatus which is constructed by using such a photoconductive type liquid crystal light valve.
In FIG. 1, an output driving voltage signal of a driving voltage source 101 is supplied between one and the other electrodes, which will be explained hereinafter, of a photoconductive type liquid crystal light valve 1. As means for writing images to the light valve 1, for example, an optical fiber tube is used. The optical fiber tube is constructed by a CRT (Cathode Ray Tube) 3 to which a video signal V.sub.V is supplied through a video amplifier 2 and optical fibers 4 which are coupled to the CRT and lead an image formed on the CRT 3 to the liquid crystal light valve 1. An output image of the CRT 3 is written to a photoconductive film of the light valve 1. The video amplifier 2 changes the level of the output video signal in accordance with a control signal V.sub.C, thereby enabling the output image of the CRT 3, namely, the writing light level to the light valve 1 is varied.
A reading light emitted from a light source 5 is reflected by a mirror 6 and passes through a condenser lens 7 and enters a polarization beam splitter 8. The P polarization component in the incident light directly passes through the polarization beam splitter 8, while the progressing direction of the S polarization component is bent at a right angle, then the S polarized light enters the light valve 1.
Now, assuming that the image is drawn in the liquid crystal layer of the light valve 1 the, P polarized component is locally included in the refection light reflected by the light valve 1 in accordance with the light and shade of the image in the liquid crystal layer. Only the P polarized component in the reflected light directly passes through the polarization beam splitter 8 and passes through a projecting lens 9, so that the image corresponding to the P polarized component is projected onto a screen 10.
FIG. 2 shows a construction of the photoconductive type liquid crystal light valve 1 which is used in such a projection type liquid crystal display apparatus.
In FIG. 2, a spacer 12 is arranged around a liquid crystal layer 11. Orientation films 13 and 14 are formed on both sides of the liquid crystal layer 11. The liquid crystal layer 11 and a photoconductive layer 15 form a laminate body so as to sandwiching a dielectric mirror 16 as a light reflecting film and a light shielding film 17 made of, for example, CdTe. The light reflecting film 16 is used to reflect the projection light which enters from the reading side. The light shielding film 17 is used to absorb the light leaked from the light reflecting film 16 and to shut out the light to the photoconductive film 15, Transparent conductive films 18 and 19 as one and another driving electrodes which sandwich such a laminate body are arranged on the outside of the liquid crystal layer 11 and photoconductive film 15. All of the above components are sealed by glass substrates 20 and 21.
In the photoconductive type liquid crystal light valve 1 having such a construction, when an image is drawn on the photoconductive film 15 by the writing light entering from the right side (writing side) in the diagram, an internal resistance of the photoconductive film 15 locally changes in accordance with the dark/light state of the image. Therefore, a driving voltage due to a driving voltage signal V.sub.LCLV which is supplied across the transparent electrodes 18 and 19 is applied to the liquid crystal layer 11 adjacent to the positions corresponding to those resistance changed portions in accordance with the light and shade of the image and is spatially modulated.
A voltage value of the driving voltage signal V.sub.LCLV is generally set in the following manner.
FIG. 3 shows characteristics of an output projection light level I to a voltage V.sub.LC [Volts] which is applied to the liquid crystal layer 11 in the case where the predetermined reading light enters the light valve 1. As will be obviously understood from the characteristics graph, when the voltage applied to the liquid crystal layer 11 is gradually increased from O V, an output projection light starts to occur at a certain voltage value. After that, the output projection light I exhibits characteristics which draw a curve i.sub.0 such that the output projection light level I passes through what is called a black level of a standard video signal and reaches a white level thereof. An applied voltage V.sub.black of the liquid crystal layer 11 corresponding to the black level in this instance indicates a threshold voltage V.sub.th. The voltage value of the output driving voltage signal V.sub.LCLV of the driving voltage source 101 is set so as to satisfy the threshold voltage V.sub.th. The threshold voltage V.sub.th is, therefore, determined by an impedance ratio of the layers between the electrodes of the light valve 1 and the voltage value of the driving voltage signal V.sub.LCLV.
Referring again to FIG. 1, however, the apparatuses excluding the screen 10, and the circuits and parts are enclosed in a casing 30 of the system. In such a casing, a temperature of locations near the light valve 1 changes due to heat sources such as lamp 5 for projecting the light, circuit board on which the video amplifier 2 and the like are installed, and the like. In accordance with such a temperature change, the voltage which is applied to the liquid crystal layer 11 is also deviated from the threshold voltage V.sub.th to be applied to the liquid crystal layer 11. The black level of the output projection light is deviated from the black level at the time of the setting of the driving voltage signal V.sub.LCLV, thereby causing deteriorations of gradation characteristics and contrast of the projection image.